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[♪ INTRO]
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From ancient times to today, volcanoes have destroyed and threatened
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countless civilizations and cities.
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And even though we have some ways of monitoring and predicting
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their activity, we still don't have a surefire way to tell
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when a volcano will erupt.
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And yet, nearly one in ten people lives within 100 kilometers
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of an active volcano.
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So, in search of more reliable ways to monitor volcanic activity,
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some scientists have started looking at volcanoes' music.
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You might not think of volcanoes as especially musical,
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but they actually do generate sound.
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It's just infrasound, whose frequency is below the threshold of human hearing.
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These sound waves can come from different phenomena, like gas
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getting pushed out of vents or out through the surface of a lava lake.
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And that moving air acts kind of like air moving through
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a wind instrument: It makes different sounds based on
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the structure it's passing through.
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Just like a horn's sound changes based on the shape and flare
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of the bell, a volcano's sound can change depending on the shape
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and depth of its crater.
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For instance, a volcano with a deep crater might produce a deep sound,
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while a volcano with a shallow crater would produce a higher-pitched sound.
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Meanwhile, narrow craters also resonate for longer periods of time,
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while wide, dish-like craters might not even resonate at all.
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And finally, the flare of the crater's rim changes the sound's timbre,
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which you can think of as the color of the sound.
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It's like how a flute and oboe sound different even when they're
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playing the same note.
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The same applies to volcanoes.
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And all this information can be useful.
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Since these sounds are so tied to the volcano's shape,
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they can change anytime the volcano changes shape.
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Which is not uncommon—especially when a volcano is getting ready to erupt.
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It all begins when the pressure of magma starts mounting beneath the surface.
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This causes lots of little earthquakes around the volcano
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that can shift around rocks and other geological features.
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The pressure can also make crater walls change their shape,
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kind of like how a balloon's shape changes as it inflates.
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Eventually, those crater walls can even fracture.
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On top of that, if there's a lava lake within the crater,
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the mounting pressure can cause it to rise.
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So, before a volcano erupts, the entire internal structure
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of the crater can be rearranged.
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And all of this changes the volcano's sound.
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One recent example of this happened in Chile in 2015.
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On March 3 of that year, Chile's Villarrica volcano erupted.
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It spewed lava and ash up to a kilometer up in the air for
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about 20 minutes—and then it all went quiet.
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As far as eruptions go, it wasn't so remarkable.
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Some property was damaged, but no one died, and the volcano
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has been quiet since.
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But the Villarrica eruption is important because it's one of
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the few cases where a volcano's sound was being monitored
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right before the eruption.
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And scientists actually observed a shift in Villarrica's sound
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on March 1, 2015, two days before the eruption.
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For one, the frequency of the sounds increased,
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so they became slightly higher-pitched.
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The sound waves also got broader, marking a change
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in the timbre of the sound.
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And finally, the volcano lost its resonance.
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Before March 1, Villarrica's sound was reverberating in the crater,
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which is shaped like an upside-down funnel.
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But after March 1, those reverberations stopped.
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That told researchers that something had changed—something inside
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the volcano was damping the sounds.
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And by the next day, new infrasound readings suggested that
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the lava lake, which was normally deep inside the funnel,
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had actually risen to nearly the rim of the crater.
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The next day, it exploded.
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At the time, scientists didn't realize that they had been listening
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to evidence of an eruption about to happen.
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They only put all the pieces together after the fact.
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But now, scientists are looking at how they might be able to use
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data like this to predict eruptions in other,
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possibly more dangerous volcanoes.
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It's tricky because there's no “normal” volcano sound.
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Every volcano has its own sound, so the only way to tell
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if something is off is to detect a change.
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And even the type of change is different from one volcano to the next.
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But by researching volcanoes for a long time, scientists
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can learn what different volcanoes sound like so they know
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a change when they hear one.
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No matter what, it's unlikely that this technique will
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completely replace traditional methods of monitoring volcanoes,
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but it's still exciting.
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This is one more tool that we can use to protect ourselves
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against dangerous volcanic eruptions.
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And there's something kind of beautiful about the fact that
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we can do that with volcano music.
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Thanks for watching this episode of SciShow!
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And if you want to learn more about volcanoes,
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you might enjoy our episode about the biggest eruptions
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we've witnessed in our history.
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You can watch that one right after this!
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[♪ OUTRO]
